It is known that 3,4,5,6-tetrahydrophthalic anhydrides (hereinafter abbreviated as ".DELTA..sup.1 -THPA") or methyl-substituted derivatives thereof are prepared usually by isomerizing 1,2,3,6-tetrahydrophthalic anhydrides (hereinafter abbreviated as ".DELTA..sup.4 -THPA") or methyl-substituted derivatives thereof. Various processes for the isomerization are known, but they are still unsatisfactory in yield and the purity of the product. For example, the process in which an inorganic strong acid, such as phosphoric acid, phosphorus pentoxide, sulfuric acid or the like, is used as a catalyst has the drawback that the starting material undergoes marked oxidation during reaction, affording .DELTA..sup.1 -THPA in low yields. Another conventional process in which palladium is used as a catalyst permits a disproportionation reaction in addition to the isomerization reaction, giving large amounts of phthalic anhydride and hexahydrophthalic anhydride as by-products to present difficulties in affording the desired product with a high purity. When producing the desired product with a high quality by these known processes, the reaction must be followed by distillation, recrystallization or like purifying procedure, which results in an increase in the number of steps as well as in cost and renders the process commercially unfavorable. Further if the yield is extremely low, it will be substantially impossible to resort to purification.
We have found that .DELTA..sup.4 -THPA or a derivative thereof, when isomerized in the presence of a palladium catalyst and a specific compound, gives .DELTA..sup.1 -THPA or a derivative thereof in a very high yield with a high purity.
An object of the invention is to provide a process for preparing .DELTA..sup.1 -THPA or derivatives thereof with a high quality almost free from by-products, for example, due to disproportionation.
Another object of the invention is to provide a process for preparing .DELTA..sup.1 -THPA or derivatives thereof in high yields with an easy procedure of purification.